The present invention relates to dielectric ceramic compositions allowing sintering at low temperature and at low partial pressure of oxygen, and exhibiting a high dielectric constant, high electrical resistivity, low temperature coefficient of the dielectric constant, and low dielectric loss, whereby they are suitable for use as dielectric material for fabricating multi-layer ceramic capacitors having copper metal internal electrodes.
Recently, multi-layer ceramic capacitors have been used widely in electrical circuits of various electrical products, because such capacitance are small and have large capasitance and high reliability.
In general, multi-layer ceramic capacitors especially exhibiting large capacitance have been manufactured by cofiring barium titanate based ceramics as dielectric materials and internal electrode metals in an oxidizing atmosphere at a temperature above 1200.degree. C. However, firing under these conditions necessitates a highly expensive internal electrode metal such as platinum or palladium, because the electrode material used in such a system must not melt and must not be oxidized in the atmosphere at the firing temperature.
Therefore there have still been two demands for dielectric ceramic materials. One demand is to be able to be sintered at a temperature below 1100.degree. C. for enabling the use of relatively cheap metals such as silver-30% palladium for internal electrodes. The other demand is to be able to be sintered in reducing atmospheres and exhibit high specific resistivity for enabling the use of base metals such as nickel or copper.
U.S. Pat. No. 4,115,493 discloses non-reducing dielectric ceramic compositions of [(Ba.sub.1-x Ca.sub.x)O].sub.m (Ti.sub.1-y Zr.sub.y)O.sub.2, which can be sintered at a temperature range of 1300.degree. C. to 1400.degree. C. in an atmosphere of low partial pressure of oxygen, and which exhibit a high electrical resistivity. Since sintering temperature of this system is above 1100.degree. C., nickel metal can be used for an internal electrode in the multi-layer ceramic capacitors employing this composition as dielectric materials, but copper metal cannot be used for that. The electrical conductivity of nickel metal is lower than copper, silver, platinum and palladium, so that when nickel metal is employed as internal electrode, loss factor of the capacitor becomes large especially in high frequency.
U.S. Pat. No. 4,450,240 discloses low temperature sintering ceramic compositions of PbTiO.sub.3 -Pb(Ni.sub.1/3 Nb.sub.2/3)O.sub.3 -Pb(Mg.sub.1/2 W.sub.1/2)O.sub.3, which can be sintered at a temperature range of 900.degree. C. to 1050.degree. C. in an oxidizing atmosphere. This patent teaches that some complex perovskite compositions containing lead can be sintered at low temperature below 1100.degree. C., and exhibit a high dielectric constant and a high electrical resistivity.
Although silver or silver-palladium metals can be used for internal electrodes in the multi-layer ceramic capacitors employing this composition as dielectric materials, copper metal cannot be used, because this composition can not be sintered in a reducing atmosphere.